As the sun
sets on NASA's Phoenix Mars Lander, both literally and figuratively, mission
engineers are beginning to shut down some of the spacecraft's instruments and
heaters to conserve what little energy it has left.
The Phoenix
lander, originally slated to run for 90 days after its May 25 touchdown on
the red planet, has completed its fifth month of exploring the surface of the
Martian arctic. Over the course of its mission, the lander dug up samples of
dirt and the rock-hard ice layer underlying the surface of Mars' arctic plains
and analyzed them for signs of past potential
habitability.
But as the
Martian northern hemisphere transitions from summer to fall, the spacecraft is
generating less
and less power as the days grow shorter, reducing the hours of sunlight
reaching its solar panels.
To keep
Phoenix chugging along for as long as possible, mission controllers will
gradually shut down four survival heaters over the next few weeks, one at a
time, to conserve power. The heaters keep the lander and its instruments within
their tested operational temperature range.
"If we
did nothing, it wouldn't be long before the power needed to operate the spacecraft
would exceed the amount of power it generates on a daily basis," said
Phoenix Project Manager Barry Goldstein of NASA's Jet Propulsion Laboratory
(JPL) in Pasadena, Calif. "By turning off some heaters and instruments, we
can extend the life of the lander by several weeks and still conduct some
science."
Engineers
sent commands to disable the first heater on Tuesday. That heater warms Phoenix's robotic arm, robotic-arm camera and Thermal and Evolved-Gas Analyzer (TEGA),
which bakes samples and "sniffs" the vapors given off to help
determine the samples' composition. Shutting down this heater is expected to
save 250 watt-hours of power per Martian day.
The Phoenix team has parked the robotic arm on the ground, with its thermal and
electrical-conductivity probe (TECP) — located on the wrist of the arm — stuck
into the dirt. The TECP will continue to measure soil temperature and
conductivity (or how heat and electricity move through the surface dirt), as
well as atmospheric humidity near the surface. (The probe does not need a heater
to operate and should continue sending back data for weeks.)
The robotic
arm won't be digging
up any more dirt samples though.
"We
turn off this workhorse with the knowledge that it has far exceeded
expectations and conducted every operation asked of it," said the robotic
arm's co-investigator Ray Arvidson, of Washington University in St. Louis.
Phoenix finished scooping up all its
samples last week and mission scientists were working to analyze them before Phoenix's
time is up.
As power
levels continue to drop, Phoenix engineers will gradually turn off the other
three heaters. The second heater serves the lander's pyrotechnic initiation
unit and is expected to add four or five days to the mission's lifetime. The
third warms the lander's main camera and meteorological instruments. The
electronics that operate those instruments should generate enough heat to keep
them, and the camera, functioning for awhile.
The fourth
heater — one of two survival heaters that warm the spacecraft and its batteries
— would be shut down in a final step. This would leave only one survival heater
to run out on its own.
"At
that point, Phoenix will be at the mercy of Mars," said Chris Lewickie of
JPL and the lead mission manager.
Engineers
are also preparing for solar conjunction, when the sun is directly between Earth
and Mars. This will happen between Nov. 28 and Dec. 13 and will block radio
transmission between the spacecraft and Earth. No commands will be sent to Phoenix during that time, but downlinks from Phoenix will continue through NASA's Odyssey
and Mars Reconnaissance orbiters.
For now,
mission controllers are uncertain whether the fourth heater will be shut down
before or after conjunction.
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